Precise mass determination for the keystone sub-Neptune planet transiting the mid-type M dwarf G 9-40
Kuzuhara, M.; Tamura, M.; Kudo, T.; Amado, P. J.; Caballero, J. A.; Pallé, E.; Reiners, A.; Ribas, I.; Quirrenbach, A.; Guenther, E. W.; Henning, T.; Morales, J. C.; Zapatero Osorio, M. R.; Passegger, V. M.; Dreizler, S.; Lafarga, M.; Gandolfi, D.; Hirano, T.; Korth, J.; Hatzes, A. P.; Narita, N.; Cochran, W. D.; Persson, C. M.; Van Eylen, V.; Nowak, G.; Serrano, L. M.; Deeg, H. J.; Goffo, E.; Lam, K. W. F.; Luque, R.; Muresan, A.; Osborne, H. L. M.; Redfield, S.; Morello, G.; Esparza-Borges, E.; Murgas, F.; Fukui, A.; Parviainen, H.; Watanabe, N.; Reffert, S.; Cifuentes, C.; Kossakowski, D.; Livingston, J.; Kabath, P.; Kawauchi, K.; Kotani, T.; Albrecht, S. H.; Nixon, M. C.
United States, Spain, Japan, Germany, United Kingdom, Denmark, Italy, Czech Republic, Sweden
Abstract
Context. Despite being a prominent subset of the exoplanet population discovered in the past three decades, the nature and provenance of sub-Neptune-sized planets is still one of the open questions in exoplanet science.
Aims: For planets orbiting bright stars, precisely measuring the orbital and planet parameters of the system is the best approach to distinguish between competing theories regarding their formation and evolution.
Methods: We obtained 69 new radial velocity observations of the mid-M dwarf G 9-40 with the CARMENES instrument to measure for the first time the mass of its transiting sub-Neptune planet, G 9-40 b, discovered in data from the K2 mission.
Results: Combined with new observations from the TESS mission during Sectors 44, 45, and 46, we are able to measure the radius of the planet to an uncertainty of 3.4% (Rb = 1.900 ± 0.065 R⊕) and determine its mass with a precision of 16% (Mb = 4.00 ± 0.63 M⊕). The resulting bulk density of the planet is inconsistent with a terrestrial composition and suggests the presence of either a water-rich core or a significant hydrogen-rich envelope.
Conclusions: G 9-40 b is referred to as a keystone planet due to its location in period-radius space within the radius valley. Several theories offer explanations for the origin and properties of this population and this planet is a valuable target for testing the dependence of those models on stellar host mass. By virtue of its brightness and small size of the host, it joins L 98-59 d as one of the two best warm (Teq ~ 400 K) sub-Neptunes for atmospheric characterization with JWST, which will probe cloud formation in sub-Neptune-sized planets and break the degeneracies of internal composition models.